The investigation has convinced me that most bad putts are a result of off-center hits on the face of the putter, causing the putter face to torque or turn changing the intended line on which the ball was to roll.
Putter design has been very generic developing a very low center of gravity with most of its weight in the lower half of the putter head. By lofting the face of the putter, the force applied causes the ball to rise before it begins to roll. Hopping and bouncing can cause the ball to go off-line. Putter shafts are usually thinner and softer causing them to bend and then kick or snap, thereby varying the force applied to the ball. This can vary the length of the putt with no way of the person knowing why the putt was long or short.